One type of process for the recovery of high purity aromatic hydrocarbons such as benzene, toluene and xylenes (BTX) from various hydrocarbon feedstreams including catalytic reformate, hydrogenated pyrolysis gasoline, etc., utilizes an aromatic selective solvent to extract the aromatic hydrocarbons by liquid-liquid extraction as the primary separating step. Typically, in the practice of such processes, a hydrocarbon feed mixture is contacted in an extraction zone with an aromatic extraction solvent which selectively extracts the aromatic components from the hydrocarbon feedstock, thereby forming a raffinate phase comprising one or more non-aromatic hydrocarbons, and an extract phase comprising solvent having aromatic components dissolved therein.
The aromatic hydrocarbons are typically recovered from the extract phase, i.e., separated from the aromatic extraction solvent and further purified by one or more distillation steps. Often, extractive and/or steam distillation is employed to assist in recovering the aromatic hydrocarbons from the solvent because both methods are particularly effective as compared to other separation techniques such as simple distillation.
In many liquid-liquid extraction processes, the raffinate phase from the extraction zone is purified by water-washing. Typically, the water used for washing the raffinate phase is obtained from the aqueous phase of an overhead, or side-draw, distillate from an extract phase steam distillation column, i.e., condensed steam, in order to provide an efficient, integrated water circulation loop. The aqueous phase, which has low levels of solvent, is then passed to one or more raffinate wash columns where residual aromatic extraction solvent is recovered from the raffinate phase. Spent raffinate wash water is typically passed to a steam generator, or otherwise vaporized, along with any other solvent-containing water streams that may be present in the process to provide stripping steam which is introduced to the extract phase distillation columns as noted above.
One process for producing high purity aromatics is described in U.S. Pat. No. 3,714,033, issued to Somekh et al., and provides for the use of a liquid-liquid extraction column and a single distillation column wherein both extractive distillation and a steam stripping occur. The patent discloses the preferred use of a polyalkylene glycol solvent which can provide a high purity aromatics product.
Another process for producing high purity aromatics is described in U.S. Pat. No. 4,058,454, issued to Asselin, and provides for the use of a liquid-liquid extraction column and extractive and steam distillation in separate columns. A particularly suitable class of solvents for use in accordance with the above-identified patent is commonly referred to as the sulfolane type which can provide a high purity aromatic product.
Still another process for producing high purity aromatics is disclosed in U.S. Pat. No. 4,081,355, issued to Preusser et al., and describes a process for recovering highly pure aromatic substances from mixtures of hydrocarbons which contain, in addition to the aromatic substances, large amounts of non-aromatic substances by liquid-liquid extraction in combination with an after arranged extractive distillation whereby the liquid-liquid extraction of the starting hydrocarbon mixture is carried out to provide an extract, introducing this extract into an after arranged extractive distillation for further separating said extract whereby the sump product (extract phase) formed is drawn off and introduced into an after arranged distillation column where it is separated into an aromatic and a solvent fraction, while the head product of the extractive distillation (raffinate phase) is reintroduced into the bottom of the extractor for liquid-liquid extraction thereof, wherein there is used in both of the extracting stages, as selective solvent, morpholine and/or N-substituted morpholine in admixture with water.
In addition to the above-described liquid-liquid extraction process, some processes for separating aromatic hydrocarbons from mixtures with non-aromatic hydrocarbons have been proposed which use extractive distillation as the primary separating step. Generally, the extractive distillation processes provide higher recoveries of the heavier aromatic hydrocarbons such as C.sub.8 aromatics and lower recoveries of light aromatics such as benzene than the liquid-liquid extraction processes.
Extractive distillation is a widespread practical and useful process for separating mixtures of materials and in particular of hydrocarbons, which cannot or can only partially be separated by simple distillation based on the boiling points of their components. In contrast to the liquid-liquid extraction frequently employed for separation of this nature, extractive distillation can exhibit advantages relating to apparatus construction and process engineering. For example, extractive distillation processes typically require only two distillation columns. Furthermore, in extractive distillation the mass transfer between the solvent and the material to be extracted can be improved due to the higher temperatures employed as compared to liquid-liquid extraction. This can result in an improved loading and for the same throughout and thus, smaller amounts of solvent can be sufficient. The obtainable advantages in apparatus construction can result in considerably smaller capital costs for an extractive distillation plant compared to those of a liquid-liquid extraction plant. The operating costs can also be lower and are sometimes only about 50% of those of a corresponding liquid-liquid extraction plant.
In liquid-liquid extraction the formation of two liquid phases is a precondition for successful separation of the starting materials. Ideally, one phase of the liquid-liquid extraction process consists of the solvent and of the components of the extract and the other phase consists of the components of the raffinate. It is frequently beneficial in liquid-liquid extraction to add water to the extraction for improving the selectivity and for favoring the formation of two liquid phases. Adding water results in the requirement of separate water circuits which can contribute to the increase of the capital costs of a liquid-liquid extraction plant but which cost is often far outweighed by the benefits of employing steam distillation for solvent recovery and purification of the aromatic product.
The underlying premise for the justification for employing extractive distillation has been completely different. The solvent employed in many extractive distillation processes is anhydrous in order to eliminate the requirement of separate water circuits. The separating effect in extractive distillation is based on the change of the vapor pressures of the individual components present in the mixture to be separated in the presence of the solvent. The changes are in the direction as to increase the vapor pressure difference between the components to be separated into either the extract or into the raffinate. Thus, the raffinate can be distilled off at the top of the extractive distillation column as the lower boiling fraction. Accordingly, it has been thought that aqueous systems were unnecessary and, therefore, undesirable.
The processes disclosed in the following patents are typical of the extraction distillation processes used for aromatic hydrocarbon recovery.
U.S. Pat. No. 4,586,986, issued to Preusser et al. discloses a method for recovering pure aromatic substances from a mixture of hydrocarbons containing both aromatic and non-aromatic fractions. The input mixture is fed through an extractive stage provided with a preliminary distillation column. In the preliminary stage the aromatics-containing product is treated at a pressure up to 20 bar and a temperature up to 300.degree. C. The pressure is adjusted to a valve at which the operational temperature of the preliminary stage is higher than the pressure and temperature in the extractive stage and the heat of the vapors discharged from the preliminary stage is used for heating the extractive stage.
U.S. Pat. No. 4,664,783, issued to Preusser et al., discloses a method for the separation of aromatics from hydrocarbon mixtures, by means of extractive distillation, employing as selective solvent N-substituted morpholine, the substitutions of which display no more than 7 carbon atoms. The raffinate produced as top product of the extractive distillation is subjected to a second distillation, whereby the produced sump product with a solvent content between 20-75% by weight and a temperature between 20.degree.-70.degree. C., is led into a separation container and there separated into a heavy and a light phase. The heavy phase is then recycled into the extractive distillation column, whereas the light phase is recycled into the second distillation column.
U.S. Pat. No. 4,776,927, issued to Emmrich et al., discloses a process for the separation of aromatics from hydrocarbon mixtures through extractive distillation using N-substituted morpholine displaying substituents having no more than 7 carbon atoms as the selective solvent. Part of the solvent is delivered to the uppermost plate of the extractive distillation column and the remainder of the solvent, preferably amounting to between 10 and 40% by weight, is introduced into the extractive distillation column in at least two partial streams onto plates above the inlet for the hydrocarbon mixture. The temperature of the respective solvent partial streams is adjusted to neither exceed the temperature of the corresponding delivery plates nor fall below this temperature by more than 10.degree. C.
U.S. Pat. No. 4,595,491, issued to Berns, discloses a process for the separation of an aromatic hydrocarbon from a hydrocarbon mixture of varying aromatic content, by means of extractive distillation, employing as a selective solvent, an N-substituted morpholine, wherein the N-substituent contains up to 7 carbon atoms. In the entry product, the weight ratio of light non-aromatic hydrocarbons to heavy non-aromatic hydrocarbon should amount to at least 0.4 to 1. The light non-aromatic hydrocarbon necessary for adjustment of this ratio can be either introduced directly into the lower part of the extractive distillation column, or added to the entry product before introducing the latter to the extractive distillation column.
In view of the two types of processes described above for separating aromatic hydrocarbons from mixtures with non-aromatic hydrocarbons, i.e., the liquid-liquid extraction processes and the extractive distillation processes, improved processes are sought which can combine the beneficial aspects of the two types of processes. More specifically, improved processes are sought which incorporate extractive distillation as the primary separation step in separating the aromatic hydrocarbons from the non-aromatic hydrocarbons and which also incorporates the steam distillation aspect of the liquid-liquid extraction processes for separating the aromatic hydrocarbons from the aromatic extraction solvents. In addition, further improvements are sought whereby the entire process can be performed in as few as two distillation columns, i.e., an extractive distillation column and a steam stripping column apart from miscellaneous equipment such as water-wash columns and the like. Furthermore, it is desired to utilize the aqueous phase condensate effluents from the extractive distillation column and the steam stripping column, for one or both of, providing stripping steam in the stripping column or for use as a raffinate wash water to recover aromatic extraction solvent from the raffinate.